The present invention relates generally to devices that convert very small mechanical displacements, as small as the sub-nanometer level, into differential voltages, and vice versa.
One position sensor that has been available since early in the last century to convert mechanical displacements into differential voltages, and vice versa, is the linear variable differential transformer (LVDT). In the conventional commercially available LVDT (Part Number 50-00-005XA, Sentech Inc, North Hills, Pa.) depicted in FIG. 1, a moving, ferromagnetic core 1 differentially couples magnetic flux from a primary coil 2 to two secondary coils 3 and 4. A current is driven through the primary coil by an oscillator 5. As the position of the core changes with respect to the secondary coils, the flux coupled to the two secondaries changes. These voltages are amplified with a differential amplifier 6 and converted to a voltage proportional to the core displacement by the signal conditioning electronics 7. For small displacements, the signal is linear. The moving core is mechanically connected to the object of interest by a shaft 8. The coils are housed in a shell that also often acts as a magnetic shield 9. Because the core 1 is a magnetically soft ferromagnet, it is often desirable to shield it from external magnetic fields. The best commercially available LVDTs are limited to a spatial resolution of 2.5 nm operating in a ±500 nm range at a bandwidth of 100 Hz (Lion Precision Model AB-01, Lion Precision, St. Paul, Minn.).
Another position sensor that can convert mechanical displacements into differential voltages is one employing capacitive technology. Commercially available capacitive sensors operating in circumstances analogous to commercially available LVDTs have surpassed LVDT performance by nearly an order of magnitude despite relying on signal conditioning circuitry of the same type as that employed for LVDTs (see, for example, the Physik Instrumente Catalog, 2001 edition). Consequently, even though they are difficult to work with and substantially more costly (because of more demanding manufacturing requirements), capacitive sensors are often the components of choice in applications that require the highest measurement resolution and bandwidth.
Because of their simplicity and low cost, it is of great interest to determine the sources of the limits in LVDT resolution and how those limitations might be overcome. As set forth more fully below, we have concluded that the limit on resolution in conventional LVDT resolution is Barkhausen noise in the ferromagnetic core. Barkhausen noise the name given to sudden jumps in the magnetic state of a ferromagnetic material (Bozorth). In ferromagnetic material, defects can lead to special sites where domain walls are preferentially pinned. The domain wall can then be depinned by thermal energy or an external magnetic field. When this happens, the domain wall will jump to another metastable pinning site, causing a sudden change in the overall magnetic state of the material. In general, when LVDT cores are formed from ferromagnetic material the flux changes due to Barkhausen noise will not cancel out in a differential measurement of the two secondaries. This uncanceled noise leads directly to noise in the core position signal. Furthermore, sudden changes in the magnetic state of the core can cause changes in the sensitivity of the sensor, again leading to positional noise.
There are a number of schemes to reduce Barkhausen (and electrical) noise in conventional LVDTs, including increasing the primary coil drive current and using amorphous magnetic materials in the core (Meydan, T., et al.; Hristoforou, E., et al.; and Midgley, G. W., et al.). While increasing the drive current will increase the signal to noise in conventional electronics, it is ineffective when dealing with Barkhausen noise because it creates a larger oscillating magnetic field that can in turn dislodge pinned domain walls more easily leading to increased positional noise. Using amorphous magnetic materials has been shown to reduce Barkhausen noise to a small degree but is ultimately ineffective because Barkhausen noise is a fundamental property of ferromagnetic materials.
Although without an understanding of Barkhausen noise issues, some non-conventional LVDT designs have eliminated its effects by substituting an air core for the ferromagnetic core of the conventional LVDT. These include a sensor designed for use in a very high magnetic field described by Ellis and Walstrom (U.S. Pat. No. 4,030,085), a pin-ball machine described by Kimura (U.S. Pat. No. 4,634,126) and a variety of mechanical gauging applications described by Neff (U.S. Pat. Nos. 2,364,237 and 2,452,862) and Snow (U.S. Pat. No. 2,503,851). The gauge described by Snow used an excitation scheme where two primary coils rather than one were excited. One coil was driven at 180 degrees from the other, resulting in oscillating magnetic fields from the two primaries that tend to cancel each other out. A single air core in the center was used as a detector. This is a different excitation scheme from the one usually used by us and others. Essentially, the roles of the primaries and secondaries are reversed. One might expect that from the reciprocity theorem (see, for example, Bertram, H. N., Theory of Magnetic Recording, Cambridge Press, 1994) that the electromagnetics of the two situations are identical. However, there are also some necessary differences in the noise performance associated with the signal conditioning. In general, the response of the sensors based on air core LVDTs in the prior art was significantly less sensitive than the improved sensor described here and would not be suitable for the sub-nanometer, high speed positioning performance we have obtained. Furthermore, the sensors described in the above prior art did not make use of any of the improvements we have incorporated into our excitation and signal conditioning electronics. The best performance claimed in these air-core LVDTs is comparable to current commercially available LVDTS.
Finally conventional LVDTs are also severely limited in the presence of an external magnetic field. As the external magnetic field increases, the core saturates and the LVDT becomes ineffectual. This limitation has been addressed by a non-conventional design in which the LVDT is fabricated entirely from non-ferromagnetic material and operated near the resonance of the primary and secondary coils (U.S. Pat. No. 4,030,085). This design however shares other limitations of conventional LVDTs in its large length scale and, for this and other reasons, is even less sensitive than the conventional design.
Present and future nanotechnology depends on the ability to rapidly and accurately position small objects and tools. Current length scales in many manufacturing technologies are in the 100 nm range and shrinking. The recording head in a commercial hard disk drive, for example, has a write gap routinely less than 100 nm and has pole-tip recession controlled to tens of nanometers. The current generation of semiconductor integrated circuits uses 180 nm wide traces, with the move to 130 nm expected within two years and to 100 nm within five years (International Technology Roadmap for Semiconductors, 1999 edition). Current sensors employed to control and verify the lithographic processes used in disk drive and IC manufacturing is already only barely providing sufficient resolution for critical dimension measurements.
Recent experimental work has gone beyond these relatively large sub-micron scales. Examples include the controlled manipulation of clusters of molecules (Piner et al.) and even individual atoms (see for example, Crommie et al.). One goal of nanotechnology is to build molecular machines (Drexler, K. E.). If such devices are constructed, they will require precise positioning of individual atoms and molecules. This will require three-dimensional positional information with sub-Å precision and rapid response as even modest molecular machines in biological organisms can contain thousands of atoms.